In the production of acetylene gas, one of the waste by-products is “carbide lime”. Canadian Patent Nos. 2,213,086 and 2,296,609, and U.S. Pat. No. 6,310,129 (“the '129 patent”), which are hereby incorporated herein by reference, each describe a method for converting raw “carbide lime” waste material into a composition usable as a filler in thermoplastic resin materials. All three patents contain essentially the same disclosure, and the '129 patent will typically be referred to hereinafter, and is representative of all three. As described in the '129 patent, the method comprises screening the raw carbide lime for size reduction, drying the screened carbide lime to obtain a low moisture content and a calcium carbonate content below 25 percent, fine grinding the dried material, and classifying the fine ground particles to separate a fraction of the particulates having a desired particle size, and corresponding lower specific gravity. As described in the '129 patent, this method results in a processed carbide lime composition which comprises 70 to 85 percent by weight calcium hydroxide and 5 to 25 percent by weight calcium carbonate, wherein the calcium carbonate is in the form of surface carbonation on the calcium hydroxide.
Since the advent of the processed carbide lime filler composition described in the '129 patent, certain benefits and drawbacks of the processed carbide lime composition have been discovered. One very significant beneficial quality discovered is that when the processed carbide lime composition is used as a filler and mixed with a resin to produce resin molded products, the composition imparts sterilizing properties to the molded product. It is known that calcium hydroxide, a main constituent of the carbide lime filler composition, has antibacterial, anti-viral, and anti-fungal properties which can last up to a hundred years. Resin molded products made using the processed carbide lime filler compositions apparently benefit from the calcium hydroxide content and are essentially sterilized. This characteristic makes resin molded products made with the processed carbide lime compositions especially useful in applications where sterilization is important.
Another benefit imparted to polyvinyl chloride (PVC) resin molded products made with the processed carbide lime filler composition is the ability of calcium hydroxide to neutralize toxic gases from PVC combustion. Calcium hydroxide reacts favourably with toxic chloride gas produced by PVC combustion, resulting in two harmless substances, i.e., water and salt. A complementary quality is that the carbide lime filler composition is also flame retardant due to its high melting point, and exhibits a “hard char” effect after burning. Consequently, coating for electrical wiring, for example, which is made from a resin containing the processed carbide lime filler composition, will not simply decompose when subjected to fire. Instead, the coating will form a hard coating on the wire, thus potentially preventing further hazard.
As explained in the '129 patent, commercial “synthetic” calcium hydroxide has been used, or the use thereof has been investigated, as fillers in resin molding systems. Some limitations and shortcomings of the use of calcium hydroxide as a filler are also described in that patent. The use of calcium carbonate as a filler material in resin molding systems is also known. There are numerous processes known for producing calcium carbonate, as described in, for example, U.S. Pat. Nos. 6,458,335 and 6,475,459 (“the '459 patent”) which are hereby incorporated by reference. The '459 patent, for example, describes a process for producing precipitated calcium carbonate, and explains that the use of precipitated calcium carbonate is growing in various industries, such as paper, plastic, and pharmaceutical industries. The process for preparing calcium carbonate particles described in the '459 patent comprises reacting a starting material containing calcium oxide with carbonate ions in the presence of water to produce calcium carbonate, and recovering the calcium carbonate, characterized in that the formation is carried out directly from calcium oxide to calcium carbonate without intermediate stages. Additionally, the process is carried out under intensive agitation such that the calcium carbonate becomes detached from the surface of the calcium hydroxide. Other methods known in the art for the manufacture of particles of calcium carbonate are described in numerous patent applications listed in the '459 patent.
Calcium carbonate does not exhibit, and does not impart, the aforementioned anti-microbial or flame retardant properties to resin molded products. Unmodified calcium hydroxide without the calcium carbonate surface coating has not been found to be as useful as the coated calcium hydroxide for application as a filler for resin molded products. This is apparently due to the surface carbonation providing a protective coating on the calcium hydroxide particles which permits the particles to be incorporated into the resin matrix in a manner in which unmodified calcium hydroxide cannot.
U.S. Pat. No. 7,883,681 (“the '681 patent”) which is hereby incorporated by reference, discloses the use of calcium carbonate coated calcium hydroxide particles as a filler material in polyvinylchloride (pvc) resin molded products. These particles were found to combine the advantage of the calcium carbonate of being readily incorporated in PVC resin while retaining the advantageous properties of calcium hydroxide including the antimicrobial and flame retardant properties. In particular the '681 patent discloses a method of making calcium carbonate coated calcium hydroxide particles comprising 70-80% by weight of calcium hydroxide and about 5-25% by weight of calcium carbonate in the form of a coating on the surface of the calcium hydroxide particles.
The '681 patent teaches a method of making the carbonate coated calcium hydroxide particles comprising suspending the calcium hydroxide particles in the air; carbonating said suspended particles by exposure to carbon dioxide; and carrying out the carbonation reaction for a predetermined period of time to create a surface carbonation on the calcium hydroxide in the proportions described above. The '681 patent describes using a gas, such as air, containing carbon dioxide as the carbon dioxide source, and further describes blowing the gas into a vessel containing the suspended calcium hydroxide particles, to create the surface carbonation on the suspended particles. This is an impinging process. Accordingly, the amount of carbon dioxide is controlled by the length of time of exposure to the gas containing carbon dioxide. In a particular embodiment, the '681 patent teaches the use of the exhaust gas of calcining ovens which contains carbon dioxide as a by product of the processes used to obtain the calcium oxide particles that are then hydrated to produce the calcium hydroxide particles.
One issue associated with the method of the '681 patent is that there is limited control over the amount of carbon dioxide exposure of the calcium hydroxide particles. As a result it may be difficult to control in an accurate manner, the amount of surface carbonation of the calcium hydroxide particles this may result in poor reproducibility and inconsistent product particles.
A further issue associated with the method described in the '681 patent is that suspending the calcium hydroxide particles in air requires a high velocity and turbulent conditions in the system. In such a system the calcium hydroxide particles will collide with one another resulting in particles of uneven shape and size. Furthermore, the flow of the carbon dioxide across the suspended particles may results in an uneven coating of calcium carbonate on the surface giving a “comet-like” shape to the coated particles having a rounded side at one face and a tail at the opposite face. Furthermore, the reaction described in the '681 patent occurs in turbulent air streams at high pressure, with carbon dioxide gas impingement causing particle size reduction due to attrition milling. In this method the aspect ratio of the particles is reduced and shear of individual particles may uncover more calcium hydroxide surface which may be detrimental to incorporation into resins.
Furthermore, the use of the by-product gas of the calcining process as described in the '681 patent may be difficult to control, as the exact amount of carbon dioxide in the by-product gas is not known. Furthermore, this method ties the processes together such that they must be done in tandem and in close proximity which may be limiting depending on the source of materials.